In Tritium (T), the number of protons (P) and neutrons (N) respectively are

1. Structure of the Atom: Protons, Neutrons, and Electrons (basic)

Imagine you are zooming into a piece of gold or a breath of air. You eventually reach the atom, the fundamental building block of all matter Science, Class VIII, Particulate Nature of Matter, p.115. While atoms are incredibly small—so small they cannot be seen even with ordinary microscopes—they are not solid, indivisible spheres. Instead, they have a complex internal structure made of three primary subatomic particles: protons, neutrons, and electrons. At the very center of the atom lies the nucleus. This dense core contains protons, which carry a positive electric charge, and neutrons, which are electrically neutral (no charge). The number of protons is the 'identity card' of an element, known as the Atomic Number (Z). For example, any atom with 7 protons is nitrogen Science, Class X, Carbon and its Compounds, p.60. The total number of protons and neutrons combined is called the Mass Number (A), representing the bulk of the atom's weight. Surrounding the nucleus are the electrons. These are tiny, negatively charged particles that move in the space around the nucleus. In a stable, neutral atom, the number of electrons is exactly equal to the number of protons, so the positive and negative charges cancel each other out. These electrons are the ones involved in 'charging' objects; for instance, when clouds rub together, charges separate, leading to the static electricity we see as lightning Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.91.

Particle	Charge	Location	Role
Proton	Positive (+)	Inside Nucleus	Defines the element (Atomic Number)
Neutron	Neutral (0)	Inside Nucleus	Adds mass to the atom
Electron	Negative (-)	Outside Nucleus	Responsible for chemical bonding

Sources: Science, Class VIII (NCERT 2025), Particulate Nature of Matter, p.115; Science, Class X (NCERT 2025), Carbon and its Compounds, p.60; Science, Class VIII (NCERT 2025), Pressure, Winds, Storms, and Cyclones, p.91

2. Atomic Number (Z) and Mass Number (A) (basic)

To understand the heart of an atom, we must look at two fundamental numbers: the Atomic Number (Z) and the Mass Number (A). The Atomic Number (Z) is the most defining characteristic of an element; it represents the number of protons found in the nucleus. This number acts like a unique chemical fingerprint—no two different elements can have the same atomic number. For instance, any atom with 6 protons is always Carbon, and any atom with 1 proton is always Hydrogen.

While the atomic number tells us who the element is, the Mass Number (A) tells us how heavy its nucleus is. Since the mass of electrons is negligible, the total mass of an atom is essentially the sum of its protons and neutrons (collectively called nucleons). You can calculate the number of neutrons in an atom by simply subtracting the atomic number from the mass number (n = A - Z). As we see in Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66, these masses are measured in atomic mass units (u).

A fascinating aspect of atomic physics is that while the number of protons (Z) for an element is fixed, the number of neutrons can vary. These different versions of the same element are called isotopes. A perfect example is Hydrogen:

• Protium (H-1): 1 proton, 0 neutrons (A = 1, Z = 1)
• Deuterium (H-2): 1 proton, 1 neutron (A = 2, Z = 1)
• Tritium (H-3): 1 proton, 2 neutrons (A = 3, Z = 1)

Term	Symbol	Represents	Formula
Atomic Number	Z	Number of Protons	Z = Protons
Mass Number	A	Protons + Neutrons	A = Z + n

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66

3. Nuclear Stability and Radioactivity (intermediate)

At the heart of every atom lies the nucleus, a tiny core where a battle of forces determines the fate of the element. For a nucleus to be stable, the Strong Nuclear Force (which acts like 'nuclear glue' between all nucleons) must overcome the Electrostatic Repulsion (the 'push' between positively charged protons). When the balance between protons and neutrons is skewed—often when the nucleus is too large or the neutron-to-proton ratio is off—the nucleus becomes radioactive. Radioactivity is the process by which an unstable nucleus spontaneously disintegrates to reach a lower, more stable energy state by emitting particles or energy Environment, Shankar IAS Academy, Environmental Pollution, p.82. This process is not just a laboratory phenomenon; it is the engine of our planet. In fact, radioactive decay in the Earth's crust and mantle provides more than half of the Earth's total internal heat Physical Geography by PMF IAS, Earth's Interior, p.58.

To understand this stability, we look at three fundamental numbers: the Atomic Number (Z) (the number of protons), the Mass Number (A) (the sum of protons and neutrons), and the Neutron Number (N). Consider the isotopes of Hydrogen. While Protium and Deuterium are stable, Tritium (³H) is radioactive because its nucleus contains 1 proton and 2 neutrons. This specific arrangement (N/Z ratio) makes it unstable, leading it to undergo decay. This instability is measured by Half-life—the time required for half of the atoms in a sample to decay Environment, Shankar IAS Academy, Environmental Pollution, p.83. While some isotopes decay in seconds, others last for millennia, making them persistent sources of environmental concern if not managed correctly.

Historically, humanity has harnessed this energy, transitioning from early theoretical understanding to building massive infrastructure. In India, for instance, nuclear energy capacity has grown from essentially zero in the 1960s to over 6.7 thousand MW by 2018–19 Geography of India, Majid Husain, Energy Resources, p.18. This power comes from controlled nuclear fission, mimicking the natural decay processes but at a highly accelerated and contained rate.

Isotope	Protons (Z)	Neutrons (N)	Mass Number (A)	Stability
Protium (¹H)	1	0	1	Highly Stable
Deuterium (²H)	1	1	2	Stable
Tritium (³H)	1	2	3	Radioactive

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.82-83; Physical Geography by PMF IAS, Earth's Interior, p.58; Geography of India, Majid Husain, Energy Resources, p.18

4. Nuclear Fusion and Hydrogen Isotopes in Technology (intermediate)

To understand the powerhouse of the universe—nuclear fusion—we must first look at the simplest element: Hydrogen. In its standard form (Protium), hydrogen has just one proton and no neutrons. However, nature provides two other "flavors" or isotopes that are critical for energy technology: Deuterium and Tritium. An isotope is simply a version of an atom with the same number of protons (which defines the element) but a different number of neutrons. Tritium (³H), for instance, contains one proton and two neutrons, giving it a mass number of three. This specific configuration makes Tritium radioactive and highly valuable as a fuel for future fusion reactors.

Nuclear fusion is the process where two light atomic nuclei combine to form a heavier nucleus, such as two hydrogen isotopes fusing into a single helium atom. This process releases a staggering amount of energy, far exceeding what we get from chemical fuels like coal or petroleum Fundamentals of Human Geography, International Trade, p.81. However, achieving fusion is incredibly difficult because nuclei are positively charged and repel each other. To overcome this repulsion, the fuel must be heated to millions of degrees Celsius—a state where it becomes a plasma Physical Geography by PMF IAS, The Universe, p.9. In the core of a star, the massive inward push of gravity provides the necessary pressure to sustain this reaction Physical Geography by PMF IAS, The Universe, p.11. Because Earth lacks this immense mass, fusion does not occur naturally in our planet's interior Physical Geography by PMF IAS, Earths Interior, p.59.

Isotope	Protons	Neutrons	Common Use/Context
Protium (¹H)	1	0	Most common form; used in Green Hydrogen production.
Deuterium (²H)	1	1	Found in "Heavy Water"; used as a moderator in nuclear reactors.
Tritium (³H)	1	2	Radioactive fuel for experimental fusion reactors (like ITER).

In modern energy strategy, hydrogen is often discussed as "Green Hydrogen" when produced via renewable energy, with the goal of reducing fossil fuel imports and carbon emissions Environment by Shankar IAS Academy, Renewable Energy, p.297. While Green Hydrogen is used for chemical energy (burning it), the fusion of Deuterium and Tritium represents the "Holy Grail" of physics—potentially providing nearly limitless, clean, and safe nuclear energy by mimicking the power of the sun on Earth.

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.11; Physical Geography by PMF IAS, Earths Interior, p.59; Environment by Shankar IAS Academy, Renewable Energy, p.297; Fundamentals of Human Geography, Class XII NCERT, International Trade, p.81

5. Understanding Isotopes, Isobars, and Isotones (intermediate)

To understand the variations in atomic structures, we must first look at the atomic nucleus. This tiny, positive core contains protons and neutrons, which together determine the identity and mass of an element Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100. We define an atom using two key numbers: the Atomic Number (Z), which is the number of protons, and the Mass Number (A), which is the sum of protons and neutrons.

Isotopes are atoms of the same element that have the same atomic number but different mass numbers. Because they have the same number of protons (and thus the same number of electrons), their chemical properties remain remarkably similar Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. A classic example is the hydrogen family. While standard hydrogen (Protium) has zero neutrons, Tritium (H-3) is a radioactive isotope of hydrogen with a nucleus consisting of 1 proton and 2 neutrons. This gives it a mass number of 3 (1+2), distinguishing it from Deuterium (1 proton, 1 neutron).

Moving beyond the same element, we encounter Isobars and Isotones. Isobars are atoms of different elements that share the same Mass Number (A) but have different atomic numbers. Conversely, Isotones are atoms that have the same number of neutrons (calculated as A minus Z) but differ in their proton count. Understanding these relationships is crucial because radioactive nuclides, like Tritium, have specific decay rates and half-lives that determine their behavior in the environment Environment, Shankar IAS Academy, Environmental Pollution, p.83.

Term	Same...	Different...	Example
Isotopes	Protons (Z)	Neutrons / Mass (A)	Protium (H-1) & Tritium (H-3)
Isobars	Mass Number (A)	Atomic Number (Z)	Argon-40 & Calcium-40
Isotones	Neutrons (A-Z)	Atomic Number (Z)	Carbon-14 & Oxygen-16 (both have 8n)

Sources: Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.67; Environment, Shankar IAS Academy, Environmental Pollution, p.83

6. The Isotopes of Hydrogen: Protium, Deuterium, and Tritium (exam-level)

In the vast world of chemistry, hydrogen is unique. While most elements are identified by their names regardless of their neutron count, the isotopes of hydrogen are so distinct in their behavior and mass that they have their own specific names: Protium, Deuterium, and Tritium. To understand them, we look at the atomic number (Z), which is 1 for all three (meaning they each have exactly one proton), and the mass number (A), which varies based on the number of neutrons in the nucleus.

Protium (¹H) is the most common form, making up over 99.98% of all hydrogen found in nature. It consists of a single proton and no neutrons. Moving up the scale, Deuterium (²H), often called "heavy hydrogen," contains one proton and one neutron. This doubling of mass significantly alters its physical properties, such as boiling and melting points, even though its chemical identity remains the same as standard hydrogen. This is a classic example of how physical properties show a gradation as molecular mass increases, while chemical properties—determined by the electron configuration—remain similar Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.67.

The third and rarest isotope is Tritium (³H). In the nucleus of a tritium atom, you will find one proton and two neutrons. This specific configuration makes tritium unstable and radioactive. Unlike the other two, tritium is not typically found in large quantities in nature; it is produced in the upper atmosphere by cosmic rays or artificially in nuclear reactors. Because it is radioactive, it is a critical component in nuclear fusion processes, often used as a fuel source alongside lithium in advanced nuclear technology Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83.

Isotope	Protons (Z)	Neutrons (N)	Mass Number (A)	Nature
Protium	1	0	1	Stable (Abundant)
Deuterium	1	1	2	Stable (Heavy)
Tritium	1	2	3	Radioactive

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.67; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of atomic structure and the concept of isotopes, this question serves as a direct application of those building blocks. It is crucial to remember that an element's identity is defined by its atomic number (Z), which is the number of protons in the nucleus. Since Tritium is an isotope of Hydrogen, it must follow the rule that all Hydrogen atoms have exactly 1 proton. As explained in the LBL ABC Wallchart, the variation between isotopes lies solely in their mass number (A), which represents the total sum of protons and neutrons.

To arrive at the correct answer, follow the deductive logic of a seasoned aspirant: the '3' in Tritium (H-3) signifies its mass number. By applying the standard formula Mass Number (3) - Atomic Number (1) = Number of Neutrons, you quickly determine there are 2 neutrons. Therefore, the correct composition is 1 P and 2 N, making (B) the only logically sound choice. This step-by-step subtraction is a foolproof way to navigate nuclear chemistry questions in the Prelims.

UPSC designed the other options to mirror the different isotopes of Hydrogen, which is a common trap for the unprepared. Option (A) refers to Protium (the most common form of Hydrogen with 0 neutrons), while Option (D) is a classic distractor meant to confuse Tritium with Helium-3. By keeping the sequence of Protium (1,0), Deuterium (1,1), and Tritium (1,2) clear in your mind, you can easily filter out these distractors and identify the specific configuration required by the question.